In recent years, with the progress of small-sized, lightweight electronic devices, printed wiring boards used for such devices have been required to have high-density wiring, which is achieved through, for example, stacking of a large number of thin layers or formation of a micro-wiring pattern. Regarding the realization of high-density wiring in such printed wiring boards, the boards are required to have low thermal expansion coefficient, for the purpose of improving the reliability of micro-wiring patterns on the boards. Particularly when such a wiring board is used as a package substrate, which is a high-density printed wiring board on which a semiconductor device such as a semiconductor chip has been mounted, strict requirements are imposed on the properties of the wiring board, as compared with the case of, for example, a core substrate.
For mounting of a semiconductor device, a flip-chip bonding process has been widely used, in place of a conventional wire bonding process. In the flip-chip bonding process, a wiring board and a semiconductor device are connected by means of a solder ball in place of a wire, which is used in the wire bonding process, to thereby mount the device on the wiring board. This mounting process may be applied to semiconductor packages such as CSP (chip scale package), PoP (package on package), and SiP (system in package).
When an electronic component is connected to a wiring board by means of solder balls, the solder balls and the wiring board are heated to about 300° C. during solder reflow. Generally, the wiring board is formed of a laminate including a metal foil, and a resin-coated film or a prepreg formed from a resin composition and a fibrous substrate or a support, wherein a wiring pattern is formed from the metal foil. Therefore, there may arise problems in that the wiring board expands due to heat; warpage occurs in the wiring board due to the difference in thermal expansion coefficient between the resin forming the wiring board and the electronic component (in particular, a semiconductor device) mounted on the wiring board; and stress is concentrated particularly on the solder balls, which connect the semiconductor device and the wiring board, resulting in cracking and connection failure.
Warpage in a semiconductor package having a PoP structure will be specifically described with reference to FIG. 1. This semiconductor package includes a substrate 18 and a wiring board provided thereon via solder balls 22, wherein the wiring board includes a semiconductor package substrate 16 having through-holes 20, a semiconductor chip 10 electrically connected to the substrate 16 by means of bonding wires 14, and a sealing material 12 provided on the semiconductor chip 10. When heat is applied to the wiring board having this structure, warpage occurs due to the difference in thermal expansion coefficient between the sealing material 12, the chip 10, and the semiconductor package substrate 16, resulting in cracking C.
Under such a circumstance, demand has arisen for a laminate which is less likely to warp and has low thermal expansion coefficient. Conventionally, laminates are generally produced by stacking a plurality of prepregs which have been prepared by impregnation of a glass woven or nonwoven fabric with a resin composition containing an epoxy resin as a main component, followed by drying, and providing a metal foil on one surface or both surfaces of the thus-stacked prepregs, followed by heating and pressurization. Although an epoxy resin is well balanced in terms of, for example, insulating property, heat resistance, and cost, the resin exhibits high thermal expansion coefficient. Therefore, as disclosed in, for example, Patent Document 1, generally, the thermal expansion coefficient of a resin composition is reduced through addition of an inorganic filler such as silica.
The thermal expansion coefficient of the resin composition can be further reduced through incorporation of a larger amount of an inorganic filler. However, when the resin composition is used for producing a multi-layer wiring board or a package substrate, a limitation is imposed on the amount of the inorganic filler incorporated, since the inorganic filler may cause, for example, moisture absorption, poor insulation reliability, adhesion failure between the resin and a wiring layer, and deterioration of drillability.
Patent Document 2 or 3 discloses a technique for reducing the thermal expansion coefficient of a resin composition by increasing the crosslink density and Tg of the resin composition. However, there is a limitation on the maximum crosslink density, since the technique for increasing crosslink density (i.e., the technique for shortening chains of cross-linked molecules) may cause problems in terms of reactivity and resin structure.
Meanwhile, Patent Document 4 discloses an effective technique for reducing the thermal expansion coefficient of a resin composition by employing an epoxy resin having an appropriate molecular weight of a segment between cross-linking points and having a polycyclic structure. However, conventional epoxy resins having a polycyclic structure exhibit low solubility in a solvent due to crystallization of the polycyclic structure caused by intermolecular attraction. Therefore, even when the epoxy resin is dissolved in an organic solvent through heating, the resin is recrystallized after having been cooled to ambient temperature.
Patent Document 5 discloses an effective technique for reducing warpage by employing a resin having a polycyclic structure. Patent Document 5 describes that a resin having a polycyclic structure is effectively employed as a sealing material. In the case where such a resin is employed as a sealing material, the resin is not required to be formed into a varnish, and thus the resin does not cause a problem in terms of recrystallization, which would otherwise occur when the resin is dissolved in an organic solvent.
However, in consideration that a resin having a polycyclic structure is used for producing a laminate, the resin must be dissolved in a solvent to form a varnish immediately before production of the laminate, since, as described above, the resin is very hard to dissolve in an organic solvent, and a varnish formed from the resin exhibits poor storage stability at ambient temperature.
Thus, improvement of the storage stability of a resin having a polycyclic structure at ambient temperature is of great industrial significance, since a varnish containing the resin exhibits improved workability during use thereof.